Power Quality Improvement at Distribution Level for Grid Connected Renewable Energy Sources

S.Syed Ahmed Int. Journal of Engineering Research and Applications www.ijera.com
ISSN : 2248-9622, Vol. 4, Issue 9( Version 5), September 2014, pp.41-45
www.ijera.com 41 | P a g e
Power Quality Improvement at Distribution Level for Grid Connected Renewable Energy Sources S. Syed Ahmed *, N. Sreekanth **, K. Siva Kumar ***
*(M.Tech, Department of Electrical Engg, GLBC College, Kadapa)
**(Associate Professor, Department of Electrical Engg, GLBC college, Kadapa)
***(Associate Professor, Department of Electrical Engg, GLBC college, Kadapa) ABSTRACT The non-linear load current harmonics may result in voltage harmonics and can create a serious PQ problem in the power system network. Active power filters (APF) are extensively used to compensate the load current harmonics and load unbalance at distribution level. This results in an additional hardware cost. However, in this project it has incorporated the features of APF in the conventional inverter interfacing renewable with the grid, without any additional hardware cost. Here, the main idea is the maximum utilization of inverter rating which is most of the time underutilized due to intermittent nature of RES. The grid-interfacing inverter can effectively be utilized to perform the four important functions they are to transfer active power harvested from the renewable resources (wind, solar, etc.), load reactive power demand support, current harmonics compensation at PCC and current unbalance and neutral current compensation in case of 3-phase 4-wire system. Moreover, with adequate control of grid-interfacing inverter, all the four objectives can be accomplished either individually or simultaneously. The PQ constraints at the PCC can therefore be strictly maintained within the utility standards without additional hardware cost. With such a control, the combination of grid-interfacing inverter and the 3-phase 4-wire linear/non-linear unbalanced load at point of common coupling appears as balanced linear load to the grid. This new control concept is demonstrated with extensive MATLAB/Simulink simulation studies
Index Terms—Active power filter (APF), distributed generation (DG), distribution system, grid interconnection, power quality (PQ), renewable energy.
I. INTRODUCTION
Electric utilities and end users of electric power are becoming increasingly concerned about meeting the growing energy demand. Seventy five percent of total global energy demand is supplied by the burning of fossil fuels. But increasing air pollution, global warming concerns, diminishing fossil fuels and their increasing cost have made it necessary to look towards renewable sources as a future energy solution. Since the past decade, there has been an enormous interest in many countries on renewable energy for power generation. The market liberalization and government’s incentives have further accelerated the renewable energy sector growth. Renewable energy source (RES) integrated at distribution level is termed as distributed generation (DG). The utility is concerned due to the high penetration level of intermittent RES in distribution systems as it may pose a threat to network in terms of stability, voltage regulation and power-quality (PQ) issues. Therefore, the DG systems are required to comply with strict technical and regulatory frameworks to ensure safe, reliable and efficient operation of overall network. With the advancement in power electronics and digital control technology, the DG systems can now be actively controlled to
enhance the system operation with improved PQ at PCC. However, the extensive use of power electronics based equipment and non-linear loads at PCC generate harmonic currents, which may deteriorate the quality of power. Generally, current controlled voltage source inverters are used to interface the intermittent RES in distributed system. Recently, a few control strategies for grid connected inverters incorporating PQ solution have been proposed. In an inverter operates as active inductor at a certain frequency to absorb the harmonic current. But the exact calculation of network inductance in real-time is difficult and may deteriorate the control performance. A similar approach in which a shunt active filter acts as active conductance to damp out the harmonics in distribution network is proposed, a control strategy for renewable interfacing inverter based on – theory is proposed. In this strategy both load and inverter current sensing is required to compensate the load current harmonics. The non- linear load current harmonics may result in voltage harmonics and can create a serious PQ problem in the power system network. Active power filters (APF) are extensively used to compensate the load current harmonics and load unbalance at distribution level. This results in an additional hardware cost. However,
RESEARCH ARTICLE OPEN ACCESS

in this paper authors have incorporated the features of
APF in the, conventional inverter interfacing
renewable with the grid, without any additional
hardware cost. Here, the main idea is the maximum
utilization of inverter rating which is most of the time
underutilized due to intermittent nature of RES. It is
shown in this paper that the grid-interfacing inverter
can effectively be utilized to perform following
important functions: 1) transfer of active power
harvested from the renewable resources (wind, solar,
etc.); 2) load reactive power demand support; 3)
current harmonics compensation at PCC; and 4)
current unbalance and neutral current compensation
in case of 3-phase 4-wire system. Moreover, with
adequate control of grid-interfacing inverter, all the
four objectives can be accomplished either
individually or simultaneously. The PQ constraints at
the PCC can therefore be strictly maintained within
the utility standards without additional hardware cost.
II. SYSTEM DESCRIPTION
The proposed system consists of RES connected
to the dc-link of a grid-interfacing inverter as shown
in Fig. 1. The voltage source inverter is a key element
of a DG system as it interfaces the renewable energy
source to the grid and delivers the generated power.
The RES may be a DC source or an AC source with
rectifier coupled to dc-link. Usually, the fuel cell and
photovoltaic energy sources generate power at
variable low dc voltage, while the variable speed
wind turbines generate power at variable ac voltage.
Fig.1. Schematic of proposed renewable based
distributed generation system
Thus, the power generated from these renewable
sources needs power conditioning (i.e., dc/dc or
ac/dc) before connecting on dc-link. The dc-capacitor
decouples the RES from grid and also allows
independent control of converters on either side of
dc-link.
A. DC-Link Voltage and Power Control Operation
Due to the intermittent nature of RES, the
generated power is of variable nature. The dc-link
plays an important role in transferring this variable
power from renewable energy source to the grid. RES
are represented as current sources connected to the
dc-link of a grid-interfacing inverter.
Fig. 2. DC-Link equivalent diagram
Fig. 2 shows the systematic representation of
power transfer from the renewable energy resources
to the grid via the dc-link.
B. Control of Grid Interfacing Inverter
In the control diagram of grid-interfacing
inverter for a 3-phase 4-wire system, the fourth leg of
inverter is used to compensate the neutral current of
load. The main aim of proposed approach is to
regulate the power. While performing the power
management operation, the inverter is actively
controlled in such a way that it always draws/
supplies fundamental active power from/ to the grid.
If the load connected to the PCC is non-linear or
unbalanced or the combination of both, the given
control approach also compensates the harmonics,
unbalance, and neutral current.
The duty ratio of inverter switches are varied in a
power cycle such that the combination of load and
inverter injected power appears as balanced resistive
load to the grid. The regulation of dc-link voltage
carries the information regarding the exchange of
active power in between renewable source and grid.
Thus the output of dc-link voltage regulator results in
an active current.
III. SIMULATION RESULTS
In order to verify the proposed control approach
to achieve multi-objectives for grid interfaced DG
systems connected to a 3-phase 4-wire network, an
extensive simulation study is carried out using
MATLAB/Simulink. A 4-leg current controlled
voltage source inverter is actively controlled to
achieve balanced sinusoidal grid currents at unity
power factor (UPF) despite of highly unbalanced
nonlinear load at PCC under varying renewable
generating conditions. A RES with variable output
power is connected on the dc-link of grid-interfacing
inverter. An unbalanced 3-phase 4-wire nonlinear
load, whose unbalance, harmonics, and reactive
power need to be compensated, is connected on PCC.
The waveforms of grid voltage, grid currents
unbalanced load current and inverter currents are
shown in Fig. 4. The corresponding active-reactive
powers of grid, load and inverter are shown in Fig. 5.

Positive values of grid active-reactive powers and inverter active-reactive powers imply that these powers flow from grid side towards PCC and from inverter towards PCC, respectively. The active and reactive powers absorbed by the load are denoted by positive signs. Fig.3. Simulation circuit for Grid Interconnection Fig.4. Simulation results: (a) Grid voltages, (b) Grid Currents (c) Unbalanced load currents, (d) Inverter Currents. Initially, the grid-interfacing inverter is not connected to the network (i.e., the load power demand is totally supplied by the grid alone). Therefore, before time s, the grid current profile in Fig. 4(b) is identical to the load current profile of Fig. 4(c). At s, the grid-interfacing inverter is connected to the network.
At this instant the inverter starts injecting the current in such a way that the profile of grid current starts changing from unbalanced non linear to balanced sinusoidal current as shown in Fig. 4(b). Fig.5.Simulation results: (a) PQ-Grid, (b) PQ-Load, (c) PQ-Inverter, (d) dc-link voltage. The corresponding change in the inverter and grid currents can be seen from Fig. 4. The active and reactive power flows between the inverter, load and grid during increase and decrease of energy generation from RES can be noticed from Fig. 5. The dc-link voltage across the grid- interfacing inverter (Fig. 5(d)) during different operating condition is maintained at constant level in order to facilitate the active and reactive power flow. Thus from the simulation results, it is evident that the grid- interfacing inverter can be effectively used to compensate the load reactive power, current unbalance and current harmonics in addition to active power injection from RES. This enables the grid to supply/ receive sinusoidal and balanced power at UPF.
IV. CONCLUSION
This paper has presented a novel control of an existing grid interfacing inverter to improve the quality of power at PCC for a 3-phase 4- wireDGsystem. It has been shown that the grid- interfacing inverter can be effectively utilized for power conditioning without affecting its normal operation of real power transfer. The grid-interfacing inverter with the proposed approach can be utilized to: i) Inject real power generated from RES to the grid, and/or, ii) Operate as a shunt Active Power Filter (APF).
This approach thus eliminates the need for additional power conditioning equipment to improve the quality of power at PCC. Extensive MATLAB/Simulink simulation as well as the DSP based experimental results have validated the proposed approach and have shown that the grid- interfacing inverter can be utilized as a multi-function device. It is further demonstrated that the PQ

enhancement can be achieved under three different scenarios. The current unbalance, current harmonics and load reactive power, due to unbalanced and non- linear load connected to the PCC, are compensated effectively such that the grid side currents are always maintained as balanced and sinusoidal at unity power factor. Moreover, the load neutral current is prevented from flowing into the grid side by compensating it locally from the fourth leg of inverter. When the power generated from RES is more than the total load power demand, the grid- interfacing inverter with the proposed control approach not only fulfills the total load active and reactive power demand (with harmonic compensation) but also delivers the excess generated sinusoidal active power to the grid at unity power factor. REFERENCES [1] J. M. Guerrero, L. G. de Vicuna, J. Matas, M. Castilla, and J. Miret, “A wireless controller to enhance dynamic performance of parallel inverters in distributed generation systems,” IEEE Trans. Power Electron., vol. 19, no. 5, pp. 1205–1213, Sep. 2004. [2] J. H. R. Enslin and P. J. M. Heskes, “Harmonic interaction between a large number of distributed power inverters and the distribution network,” IEEE Trans. Power Electron., vol. 19, no. 6, pp. 1586– 1593, Nov. 2004. [3] U. Borup, F. Blaabjerg, and P. N. Enjeti, “Sharing of nonlinear load in parallel- connected three-phase converters,” IEEE Trans. Ind. Appl., vol. 37, no. 6, pp. 1817– 1823, Nov./Dec. 2001. [4] P. Jintakosonwit, H. Fujita, H. Akagi, and S. Ogasawara, “Implementation and performance of cooperative control of shunt active filters for harmonic damping throughout a power distribution system,” IEEE Trans. Ind. Appl., vol. 39, no. 2, pp. 556–564, Mar./Apr. 2003. [5] J. P. Pinto, R. Pregitzer, L. F. C. Monteiro, and J. L. Afonso, “3-phase 4-wire shunt active power filter with renewable energy interface,” presented at the Conf. IEEE Rnewable Energy & Power Quality, Seville, Spain, 2007. [6] F. Blaabjerg, R. Teodorescu, M. Liserre, and A. V. Timbus, “Overview of control and grid synchronization for distributed power generation systems,” IEEE Trans. Ind. Electron., vol. 53, no. 5, pp. 1398–1409, Oct. 2006.
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presently working as Associate Professor in Global College of Engineering and Technology, Kadapa. Mr. K. Siva Kumar, pursuing Ph.D from JNTUA, Anantapur. He received Post graduate from JNTUH, Hyderabad and graduate (EEE) from NBKR Institute of Science and Technology, Vakadu, Nellore District, affiliated to Sri Venkateswara University, Tirupathi. Presently working as an Associate Professor & Head of Department, Electrical and Electronics Engineering, Global College of Engineering and Technology, Andhra Pradesh, Kadapa.

Power Quality Improvement at Distribution Level for Grid Connected Renewable Energy Sources

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